The present invention relates to injection molding machines and methods and, in particular, to an injection molding system that allows for the assembly of molded parts as a stage of the molding process.
Injection molding is a manufacturing process in which heated thermoplastic is forced under pressure into a mold. After the thermoplastic cools, the mold is separated along a part line and a molded thermoplastic part is ejected. With the proper mold, complex parts can be manufactured in extremely high volumes and low per piece costs.
Many products that cannot be manufactured by injection molding in a single mold can be assembled from one or more separately molded parts. The step of assembling these parts can significantly increase the cost of the product and in certain cases decrease part volumes otherwise obtainable.
For this reason, there is considerable interest in so-called “in-mold” assembly techniques. In one such technique termed “two shot” molding, a mold having replaceable portions allows different features to be added into one changing mold cavity over several sequential steps of plastic injection. The resulting product may be a single, fused structure or, by making the two shots of plastics that resist adhesion to each other, the resulting product may be an assembled collection of movable parts.
More complex products can be created by a different technique in which separately molded parts are partially retained by a portion of the mold which is then moved within the injection molding machine to a different position for assembly with other molded parts made simultaneously in a different portion of the same machine.
In one published method using this technique henceforth referred to as “in-machine” assembly, a product is manufactured using a four-part mold having two outer sections attached to the molding machine platens, and two center sections between these outer sections and which may rotate about two offset axes both parallel to the mold clamp direction. During a first step, parts are molded in mold cavities formed by the joining of corresponding portions of pairs of the outer sections and center sections. The molded parts are then retained on the center sections which rotate to move the molded parts into opposition for assembly. The mold sections are again closed and the molded parts are assembled.
The parts may be held together by adhesive, molded-in-place rivets or other techniques applied immediately before or during the mold closure. This technique for in-mold assembly may be suitable, for example, for manufacturing products having cavities loosely holding a non-molded material.
The in-machine molding technique described above requires a shaft to be extended through each platen of the injection molding machine for rotating the revolving mold section sections such as may require substantial modification of the injection molding machine. Further the full area of the platen cannot be exploited for molding because of the need to reserve room for assembly of cooled parts. This limits the molding throughput. Finally, insertion of other parts or application of glue to the parts prior to assembly is difficult because the parts are always in-between the platens and the molds limit access to those parts.